Method of promoting growth of green algae

ABSTRACT

Provided is a method of culturing green algae which promotes the growth of the green algae which is in a state of being a green swarm cell by irradiating the green algae that accumulate astaxanthin with an artificial light. The green algae are grown in a liquid medium while maintaining a state in which the color of a culture solution of the green algae is green or brown by alternately and continuously radiating a red illumination light and a blue illumination light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2014/052488, filed Feb. 4, 2014, claiming priority based on Japanese Patent Application No. 2013-019973, filed Feb. 4, 2013, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method of promoting the growth of green algae, and particularly to a method of promoting the growth of green algae which promotes the growth of the green algae which is in a state of being a swarm cell by irradiating the green algae that accumulate astaxanthin, with artificial light.

BACKGROUND ART

It is known that astaxanthin is a kind of red carotenoid and has a strong antioxidant effect.

PTL 1 discloses a method of producing a green alga that produces astaxanthin and a method of producing a green alga containing astaxanthin in a high concentration by irradiating a green alga, the cell of which is changed to a cyst cell, with light under a certain conditions.

Green algae belonging to the genus Haematococcus have been known as green algae which accumulate astaxanthin. It has been known that the Haematococcus exists in a state of being grown as a green swarm cell under an appropriate culture and light irradiation conditions and in a state in which the swarm cell is changed to the cyst cell due to stress such as a light environment or nutritional deficiency and a significant amount of astaxanthin is accumulated in the cell.

It is commercially useful to industrially culture a large amount of green algae as raw materials for health food products, pharmaceutical products, or the like. Moreover, a method of increasing the accumulation of astaxanthin in the cyst cell as shown in PTL 1 has been studied.

On the other hand, there is a need to improve the productivity by promoting the growth of the green algae and by shortening the culture period. In this manner, in order to shorten the period required for obtaining a large amount of cells, it is useful to promote the growth of the green algae which is in a state of being the swarm cell, which is a cell prior to being changed to the cyst cell, and to produce a cell in high density in a short period of time. However, the method of promoting the growth of the green algae which is in a state of being the swarm cell is not known.

Meanwhile, a method of growing an organism by promoting photosynthesis using artificial light is known.

For example, NPL 1 discloses an effect of irradiation with red LED light and blue LED light, on the growth of the Haematococcus.

In addition, PTL 2 discloses a light source for cultivating a plant or the like by simultaneously or alternately turning on the blue LED light and the red LED light.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Application, First Publication     No. 2007-097584 -   [PTL 2] Japanese Unexamined Patent Application, First Publication     No. H08-103167

Non-Patent Literature

-   [NPL 1] Katsuda, T., et al., Enzyme and Microbial Technology     35 (2004) 81-86

SUMMARY OF INVENTION Technical Problem

None of the above-described literature discloses the method of promoting the growth of the green algae which is in a state of being the swarm cell in a liquid medium and culturing the cell in high density. Moreover, such a method is not known.

Therefore, the present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide a growth promoting method for improving the productivity by promoting the growth of the green algae which is in a state of being the swarm cell and by shortening the culture period, in order to industrially culture a large amount of the green algae that accumulate astaxanthin.

Solution to Problem

The present inventors have conducted extensive studies on the effect of promoting the growth of the green algae by radiating an artificial light. As a result, they have found that, surprisingly, the swarm cell grows in high density in a short period of time while maintaining a green stage (a state in which the color of a culture solution is green or brown during the culturing of the green algae) if red illumination light and blue illumination light are alternately and continuously radiated.

The present invention is as follows.

[1] A method of promoting the growth of green algae which promotes the growth of the green algae which is in a state of being a green swarm cell by irradiating the green algae that accumulate astaxanthin with an artificial light,

in which the green algae are grown in a liquid medium while maintaining a state in which the color of a culture solution of the green algae is green or brown by alternately and continuously radiating a red illumination light and a blue illumination light.

[2] The method of promoting the growth of green algae according to the above-described [1], in which the green algae accumulating astaxanthin are unicellular green algae belonging to the genus Haematococcus.

[3] The method of promoting the growth of green algae according to the above-described [2], in which the unicellular green algae belonging to the genus Haematococcus are Haematococcus pluvialis (H. pluvialis) or Haematococcus lacustris (H. lacustris).

[4] The method of promoting the growth of green algae according to any one of the above-described [1] to [3], in which a wavelength of the red illumination light is 570 nm to 730 nm and a central wavelength is 645 nm to 680 nm.

[5] The method of promoting the growth of green algae according to any one of the above-described [1] to [4], in which a wavelength of the blue illumination light is 400 nm to 515 nm and a central wavelength is 440 nm to 460 nm.

[6] The method of promoting the growth of green algae according to any one of the above-described [1] to [5], in which a light source of the red illumination light is an LED.

[7] The method of promoting the growth of green algae according to any one of the above-described [1] to [6], in which a light source of the blue illumination light is an LED.

[8] The method of promoting the growth of green algae according to any one of the above-described [1] to [7], in which the light amount of the red illumination light and the blue illumination light is 10 μmol/m²/s to 100 μmol/m²/s at photosynthetic photon flux density in a light irradiation surface of the culture solution.

Advantageous Effects of Invention

According to the present invention, in a method of promoting the growth of green algae that accumulate astaxanthin, a simple and excellent growth promoting method, by which it is possible to shorten the culture period and to improve productivity by promoting the growth which is in a state of being a swarm cell, is provided.

In addition, according to the growth promoting method of the present invention, it is possible to promote the growth of the green algae which is in a state of being the swarm cell and to produce a cell in high density in a short period of time. Moreover, it is possible to obtain a large amount of green algae (to grow the swarm cell) using the growth promoting method of the present invention, and then, to efficiently produce a large amount of astaxanthin by transferring the obtained green algae to the cyst cell.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described. The embodiments described below are examples of representative embodiments of the present invention and the present invention is not limited thereto. Therefore, the scope of the present invention is not interpreted narrowly.

The method of promoting the growth of green algae according to the present invention is a method of promoting the growth of a swarm cell in a liquid medium while maintaining a green stage (a state in which the color of a culture solution is green or brown during the culturing of the green algae) of green algae by alternately and continuously performing a process of radiating red illumination light (hereinafter, also referred to as “process R”) and a process of radiating blue illumination light (hereinafter, referred to as “process B”) with respect to the green algae that accumulate astaxanthin.

(Wavelength)

An example of the red illumination light used in the present invention includes light having a wavelength of 570 nm to 730 nm, and light having a wavelength of 620 nm to 730 nm is preferably used. In addition, red illumination light, which has a wavelength of 645 nm to 680 nm as a central wavelength, is more preferably used and red illumination light having a central wavelength of 660 nm is still more preferable.

An example of the blue illumination light used in the present invention includes light having a wavelength of 400 nm to 515 nm.

In addition, blue illumination light having a central wavelength of 440 nm to 460 nm is preferably used and blue illumination light having a central wavelength of 445 nm to 450 nm is more preferable.

The red illumination light and the blue illumination light can have a predetermined wavelength region by having the above-described wavelengths as central wavelengths. Examples of the wavelength regions include, for the blue illumination light, 450 nm±30 nm, preferably 450 nm±20 nm, and more preferably 450 nm±10 nm, and for the red illumination light, 660 nm±30 nm, preferably 660 nm±20 nm, and more preferably 660 nm±10 nm.

Here, the “alternately and continuously” means light irradiation in which a process R and a process B are alternately repeated without any interval between the process R and the process B during the culturing period of the green algae. However, even in a case where there is a non-irradiation period or a simultaneous irradiation period between the process R and the process B for a short period of time or in a case where a momentary non-irradiation period is provided by setting the duty ratio of LED light lower than 100% and pulse light is continuously irradiated, the light irradiation in which a light irradiation process which is substantially the same as the process R or process B of the present invention is alternately repeated and an effect which is substantially the same as the present invention can be obtained, and is included in the “alternately and continuously” of the present invention. An acceptable total duration of the non-irradiation period or the simultaneous irradiation period for a short period of time in the present invention is recommended to be within one hour at a minimum and being within 30 minutes is preferable. However, the duration varies depending on the type of the green alga, and therefore, it is possible to obtain the duration through a preliminary test or the like.

In a case where a rest period, an irradiation process with other light sources, a simultaneous irradiation process of the process R and the process B, or the like is interposed between the process R and the process B, there is a risk that it will be impossible to obtain a sufficient growth promoting effect. For this reason, it is important that the process R and the process B be alternately and continuously repeated without any interval between the process R and the process B.

In the present invention, the process R and the process B are alternately and continuously performed and a radiation cycle configured to have the process R and the process B is repeated in this manner. Whether the process R or the process B is to be performed first in the irradiation cycle is arbitrary.

(Irradiation Period)

Irradiation periods of the process R and the process B can be arbitrarily set in so far as the effect of the present invention is exhibited. The irradiation periods of the process R and the process B may be constant throughout the whole culture period and may be extended or shortened. The irradiation periods of the process R and the process B may be the same as each other and different from each other. The irradiation periods of the process R and the process B can be arbitrarily set in so far as the effect of the present invention is exhibited.

In addition, it is possible to arbitrarily set the number of processes in the culture period. The process may start from either of the process R or the process B at the time of starting the culturing and may finish with either of the process R or the process B.

A specific example of the mode includes alternate and continuous irradiation in which the process R is performed for 3 hours to 21 hours and the process B is performed for 3 hours to 21 hours.

An example of a preferred mode includes conditions in which each light amount of the red irradiation light and the blue irradiation light is 50 μmol/m²/s, and the irradiation periods of the process R and the process B are set to 16 hours and 8 hours, being constant throughout the culture period. At this time, the light amount ratio (red irradiation light:blue irradiation light) is 5:3.

(Light Amount)

The light amounts of the red irradiation light and the blue irradiation light in the process R and the process B are not particularly limited as long as the light amounts are within a range where it is suitable to grow green algae accumulating astaxanthin. For example, photosynthetic photon flux densities (PPFD) in a light irradiation surface of the culture solution are 5 μmol/m²/s to 200 μmol/m²/s, preferably 10 μmol/m²/s to 100 μmol/m²/s, and more preferably 20 μmol/m²/s to 70 μmol/m²/s. The light amounts of the red irradiation light and the blue irradiation light may be the same as or different from each other.

The “light irradiation surface” refers to a place (a place close to the light irradiation surface without limitation) which is close to the culture solution and is not being limited to a place next to the culture solution, while it is difficult to strictly define the “light irradiation surface” from the size of equipment for measuring PPFD or the shape of a culture container.

In addition, the light amount ratios of the red irradiation light and the blue irradiation light in the process R and the process B are not limited as long as the effect of the present invention can be obtained. For example, it is possible to set the range of the light amount ratio of “red:blue” to 1:20 to 20:1 and it is possible to arbitrarily set the range thereof to 1:1, 5:3, 2:1, 3:1, 4:1, 10:1, 20:1, and the like. Among these, the light amount ratio of “red:blue” is preferably within a range of 1:1 to 10:1, more preferably within a range of 3:2 to 7:1, and still more preferably within a range of 5:3 to 3:1.

(Irradiation Device)

Any device for irradiating the red irradiation light and the blue irradiation light used in the present invention can be used regardless of the shape of the device or the type of the light source as long as it is possible to perform each process of the present invention. The device includes a light irradiation unit that irradiates the green algae with the red illumination light and the blue illumination light and a control unit that alternately and continuously performs the process R and process B by controlling the light irradiation unit.

A light source for radiating red light and/or blue light is included in the light irradiation unit. It is possible to use a conventionally known light source for a light source of the red light and the blue light. It is preferable that a photo-semiconductor element such as a light emitting diode (LED) or a laser diode (LD) that radiates light, in which it is easy to select the wavelength and in which the percentage of light energy in an effective wavelength region is large, be used for a light source.

The photo-semiconductor element is small in size and has a long service life. Moreover, the photo-semiconductor element has good energy efficiency as it emits at a specific wavelength depending on the material and there is no unnecessary heat radiation. Furthermore, the photo-semiconductor element hardly disturbs a cell even if close irradiation is performed on green algae. For this reason, it is possible to perform the culturing at low electric power cost while saving space compared to other light sources using the photo-semiconductor element for a light source.

It is possible to use an SMD line light source, in which an SMD (two chips surface mount device) mounted with a combination of a red light semiconductor element and a blue light semiconductor element is arranged in a linear shape; a monochromatic line light source or a monochromatic panel light source, in which either the red light semiconductor element or the blue light semiconductor element is arranged in a linear, tubular, or planar shape; and the like, for a light source.

The semiconductor element is capable, in principle, of flicker operation at a frequency as high as several megahertz (MHz) or higher. For this reason, it is possible to switch the process between the process R and the process B at an extremely high speed using the photo-semiconductor element for a light source.

Examples of LEDs that radiate light in the above-described wavelength region include an ALGaInP light emitting diode (GaP substrate, 660 nm of a red wavelength) which is available as a product number of HRP-350F from Showa Denko K.K. or the like for a red LED; and a light emitting diode having a product number of GM2LR450G of Showa Denko K.K. or the like for a blue LED.

Examples of light sources other than the light emitting diode include a straight-tube type or a compact type fluorescent lamp and an electric bulb-type fluorescent lamp, a high-pressure discharge lamp, a metal halide lamp, and a laser diode. An optical filter may be used in order to selectively use the light in the above-described wavelength region by combining the light sources.

The control unit maintains the light amounts and/or the irradiation periods of the red illumination light and the blue illumination light, which are radiated from the light irradiation unit, to be predetermined values or changes the light amounts and the irradiation periods thereof to predetermined light amounts and irradiation periods.

The control unit can be configured by using a general computer. For example, in a case where an LED is used for a light source, the size of a drive current of an LED is adjusted based on a control pattern which is previously held and stored in a memory or a hard disk, and the light amounts and/or the irradiation periods of the red illumination light and the blue illumination light is changed. In addition, the control unit switches and drives a plurality of LEDs radiating light beams in different wavelength regions based on the control pattern and changes the wavelength region of the radiated light.

(Algae)

Green algae used in the present invention can be used without particular limitation as long as the green algae accumulate astaxanthin. For example, unicellular green algae belonging to the genus Haematococcus are preferably used.

Examples of preferred green algae include Haematococcus pluvialis (H. pluvialis), Haematococcus lacustris (H. lacustris), Haematococcus capensis (H. capensis), Haematococcus droebakensi (H. droebakensi), and Haematococcus zimbabwiensis (H. zimbabwiensis).

Examples of Haematococcus pluvialis (H. pluvialis) include a UTEX 2505 strain and a K 0084 strain.

Examples of Haematococcus lacustris (H. lacustris) include an NIES 144 strain, an ATCC 30402 strain, an ACTT 30453 strain, an IAM C-392 strain, an IAM C-393 strain, an IAM C-394 strain, an IAM C-339 strain, a UTEX 16 strain, and a UTEX 294 strain.

An example of Haematococcus capensis (H. capensis) includes a UTEX LB 1023 strain.

An example of Haematococcus droebakensi (H. droebakensi) includes a UTEX 55 strain.

An example of Haematococcus zimbabwiensis (H. zimbabwiensis) includes UTEX LB 1758 strain.

Among these, Haematococcus pluvialis (H. pluvialis) is preferably used.

(Green Stage)

A green stage in the present invention indicates a state in which the color of a culture solution is green or brown during the culturing of green algae and corresponds to a state in which a cell within a medium grows as a green swarm cell under an appropriate culture and light irradiation conditions.

The appropriate culture and light irradiation conditions indicates that red illumination light and blue illumination light having a wavelength described in the specification are radiated using an irradiation device at a light amount for an irradiation period described in the specification, and the culturing is performed under the culture condition using the algae, the medium, and the culture device described in the specification.

In contrast, a state, in which the swarm cell is changed to the cyst cell due to stress such as a light environment or nutrition deficiency and the color of the culture solution is changed to red by accumulating a significant amount of astaxanthin in the cell, is called a red stage.

At the end of the green stage while being transferred to the red stage from the green stage, a green cell (cell which maintains a state of the swarm cell) and a red cell (cell which is changed to the cyst cell) are mixed and the color of the culture solution is changed to brown. Finally, the swarm cell disappears and the stage is transferred to the red stage in which a significant amount of astaxanthin is accumulated.

In order to produce a large amount of astaxanthin, it is preferable to transfer the stage to the red stage after promoting the growth of the green algae which is in a state of being the swarm cell and producing a cell in high density in a short period of time, while maintaining the green stage, that is, a state immediately before the color of culture solution becomes red (a state in which there is a swarm cell in the medium and which includes not only a state where the color of the culture solution is green but also a state where the color of the culture solution is brown) as possible. In the growth promoting method of the present invention, it is possible to grow a cell in high density in a short period of time by realizing the above.

In addition, it is possible to grow a cell in higher density in a shorter period of time compared to the related art similarly to the swarm cell in the Haematococcus even in other green algae by applying the method of the present invention.

(Medium)

A liquid medium used in the culturing of green algae is not particularly limited. In general, it is possible to preferably use a medium containing nitrogen, trace metal inorganic salts (for example, phosphorus, potassium, magnesium, and iron), and vitamins (for example, thiamin) which are required for proliferation. For example, it is possible to use media such as an AF-6 medium, a BG-11 medium, a C medium, an MBM medium, an MDM medium, and a VT medium (the medium compositions are disclosed in the list of media in the homepage of Incorporated Administrative Agency National Institute of Health Sciences), modified media thereof, and the like. The media can be appropriately selected in accordance with the types of green algae or the purpose of culturing. For example, for the purpose of growing the green alga belonging to the genus Haematococcus, it is preferable to use the C medium or the BG-11 medium and modified media thereof. Components such as a carbon source and a nitrogen source may be appropriately added in addition to the above-described compositions.

(Culture Device)

The shape or the size of the culture device of the green algae is not particularly limited as long as the device can supply carbon dioxide and can radiate light. For example, in a case where the culturing is performed in a laboratory, it is possible to use a flat culture bottle, a conical flask, a flat-bottom flask, or the like. In a case where the culturing is performed at a pilot scale or an industrial production scale, it is possible to use a culture tank, which is provided with a device that can irradiate a transparent container made of glass or plastic with light from the outside or the inside thereof, or a culture tank, which is provided with a device that can irradiate the inside of a metallic container with light from the inside thereof. As such a culture tank, for example, a jar-type culture tank, a tube-type culture tank, an air-dome type culture tank, and a hollow cylindrical culture tank are used. In addition, in any case, an airtight container is preferably used. The tanks may be installed with a stirring machine as necessary.

(Culture Condition)

The culture condition is not particularly limited. The temperature, the pH, and the like which are generally used in the culturing of the green algae can be used. The temperature for culturing the green algae is controlled to be constant, for example, 15° C. to 35° C., and preferably 20° C. to 25° C. The pH of the culture solution is held between 4 and 10, preferably between 5 and 9, and more preferably between 6 and 8.

For the purpose of supplying the carbon source and controlling the pH, it is preferable to supply carbon dioxide. In general, mixed air containing carbon dioxide at a concentration of 1 v/v % to 10 v/v % is supplied to a medium or to the upper space of the culture tank. For example, the mixed air is supplied by being continuously circulated so as to be 0.2 vvm to 2 vvm. Alternatively, the carbon dioxide may be intermittently supplied by substituting the upper space of the culture tank with the above-described mixed air and sealing it, and then, repeating the substituting and the sealing. It is preferable that the concentration of the carbon dioxide be appropriately changed in accordance with the growth of the green algae or that the circulation amount be changed. As an example of a preferred culture condition, culturing is performed while supplying an appropriate circulation amount by changing the circulation amount so as to maintain the pH 7 in accordance with the growth of the green algae, by continuously circulating the mixed air containing carbon dioxide at a concentration of 5 v/v % so that the mixed air becomes 1 vvm, in a state where the cells of the green algae are gently stirred at a temperature of 20° C. and at the pH of 7 so as to evenly radiate light.

(Measurement of Amount of Algal Body)

It is possible to measure the growth state of the green algae using a known method such as measurement of absorbance of culture solution, fluorescence measurement, measurement using a hemocytometer, Coulter Counter, and dry weight measurement. Among these, the absorbance measurement at 560 nm, the measurement using hemocytometer, and the dry weight measurement are preferably performed.

EXAMPLES

Hereinafter, the effect of the present invention becomes clearer by the examples. The present invention is not limited to the following examples and can be implemented through appropriate modification within the scope not departing the gist of the present invention.

In the present examples, the present invention is described using examples using a Haematococcus lacustris NIES-144 strain. However, the present invention is not limited to the examples.

Example 1 Preparation of Green Alga

A modified medium of a BG-11 medium was prepared. The medium composition is shown in Table 1.

TABLE 1 Composition g/L NaNO₃ 0.45 K₂HPO₄•3H₂O 0.04 MgSO₄•7H₂O 0.075 CaCl₂•2H₂O 0.036 Citric Acid (anhydrous) 0.006 Ammonium iron(III) citrate 0.006 EDTA•2Na 0.001 Na₂CO₃ 0.02 H₃BO₄ 0.00181 MnCl₂•4H₂O 0.00022 ZnSO₄•7H₂O 0.00008 Na₂MoO₄ 0.000021 CuSO₄•5H₂O 0.079 Co(NO₃)₂•6H₂O 0.049

20 ml of a medium was input to a culture flask with a 50 ml capacity and a Haematococcus lacustris NIES-144 strain was inoculated. Photosynthesis-effective photon density (PPED) was adjusted to 30 μmol/m²/s on the irradiation surface of the culture flask using a light quantum meter (manufactured by System Instruments Co., Ltd.) and using a white fluorescent lamp for a light source; light irradiation was intermittently performed on the medium for 12 hours for an irradiation period and 12 hours for a non-irradiation period; the culture is performed for 14 days at a temperature of 20° C.; and a culture solution having a cell density of 2×10⁵ cell/ml was prepared.

(Main Culture)

20 ml of the same medium was input to a conical flask with a 100 ml capacity and 2 ml of the above-described culture solution was inoculated. The PPED was adjusted to 62.5 μmol/m²/s for red light and 37.5 μmol/m²/s for blue light on the irradiation surface of the culture solution using a light quantum meter and using 3 in 1 LED illumination (manufactured by NKsystem, wavelength: 660 nm for red light, 520 nm for green light, and 450 nm for blue light) for a light source; after irradiating (process R) the medium with red light for 12 hours, the medium was continuously irradiated with blue light for 12 hours (process B) and the process R and the process B were alternately and continuously performed; and the culture was performed for 8 days at a temperature of 20° C. and a pH of 7. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 5:3.

While supplying carbon dioxide, gas containing carbon dioxide at a concentration of 7 v/v % to 10 v/v % was ventilated to a head space of the flask through a sterilization filter and the head space was sufficiently substituted with the gas, and then, the head space was sealed with a butyl rubber plug. Each time when the plug was released at the time of sampling, the head space was substituted with the gas through the same procedure.

During the culturing, the concentration of the carbon dioxide was adjusted so that the pH of the medium became 7. Specifically, the concentration of the carbon dioxide to be supplied was adjusted so as to be 7 v/v % from day 0 to day 4 and 10 v/v % after day 4.

The cell density, the absorbance at 560 nm (A560), and the swarm cell ratio were measured by appropriately performing sampling during the culturing. The cell density was measured using a hemocytometer. The swarm cell ratio in the cell was calculated using the hemocytometer. In addition, the color of the culture solution was observed.

Example 2

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 50 μmol/m²/s for red light and 50 μmol/m²/s for blue light, the medium was continuously irradiated with the blue light for 9 hours (process B) after being irradiated with the red light for 15 hours (process R). At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 5:3.

Example 3

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 50 μmol/m²/s for red light and 50 μmol/m²/s for blue light, the medium was continuously irradiated with the blue light for 6 hours (process B) after being irradiated with the red light for 18 hours (process R), and the main culturing was performed for 6 days. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 3:1.

Example 4

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 50 μmol/m²/s for red light and 50 μmol/m²/s for blue light, the medium was continuously irradiated with the blue light for 3 hours (process B) after being irradiated with the red light for 21 hours (process R), and the main culturing was performed for 6 days. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 7:1.

Comparative Example 1

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 31.3 μmol/m²/s for red light and 18.8 μmol/m²/s for blue light and the medium was simultaneously and continuously irradiated with the red irradiation light and the blue irradiation light. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 5:3.

Comparative Example 2

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 50 μmol/m²/s using a white fluorescent lamp for a light source and the medium was continuously irradiated.

Comparative Example 3

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 37.5 μmol/m²/s for red light and 12.5 μmol/m²/s for blue light, the medium was simultaneously and continuously irradiated with the red irradiation light and the blue irradiation light, and the main culturing was performed for 6 days. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 3:1.

Comparative Example 4

The culturing was performed similarly to Example 1 except that the light irradiation condition was adjusted so as to be 43.7 μmol/m²/s for red light and 6.3 μmol/m²/s for blue light, the medium was simultaneously and continuously irradiated with the red irradiation light and the blue irradiation light, and the main culturing was performed for 6 days. At this time, the light amount ratio of the red irradiation light to the blue irradiation light per 24 hours was 7:1.

The result of Examples 1 to 4 and Comparative Examples 1 to 4 is shown as the following.

In Examples 1 and 2, A560 and the cell density were high and the growth was promoted compared to Comparative Examples 1 and 2. In all of Examples 1 and 2, the color of the culture solution was green at day 6 and was changed to brown at day 8, the swarm cell ratio was greater than or equal to 70%, and the green stage was maintained.

In contrast, in Comparative Examples 1 and 2, the color of the culture solution became red at day 8, and therefore, it was impossible to maintain the green stage.

In Examples 3 and 4, A560 and the cell density were high and the growth was promoted compared to Comparative Examples 3 and 4. In all of Examples 3 and 4, the color of the culture solution was green at day 6, the swarm cell ratio was greater than or equal to 70%, and the green stage was maintained.

In contrast, in Comparative Examples 3 and 4, while the color of the culture solution was maintained to be green at day 6, the cells were coagulated causing precipitation, and therefore, it was impossible to maintain zoospores.

TABLE 2-2 Light irradiation conditions Light quantum density Red Blue swarm (660 nm of (450 nm of Mutual Conditions Light Cell density cell wavelength) wavelength) Red Blue amount ratio (Cell density) A560 ratio μmol/m²/s μmol/m²/s (hour/day) (hour/day) Red Blue cell/ml Day 6 Day 8 (%) Example 1 65.6 37.5 12 12 6 3 3.5 × 10⁵ 0.67 1.28 80 (*1 Example 2 60 50 15 9 5 3 3.5 × 10⁵ 0.79 1.6  80 (*1 Example 3 50 50 18 6 3 1 8.3 × 10⁵ 0.93 — 70 (*2 Example 4 50 50 21 3 7 1 1.4 × 10⁶ 0.91 — 70 (*2 Comparative 31.3 18.8 Simultaneous and 5 3 2.8 × 10⁶ 0.57 0.81 80 (*1 Example 1 continuous irradiation Comparative White fluorescent — — — — 1.8 × 10⁶ 0.39 0.83 70 (*1 Example 2 lamp: 50 Comparative 37.5 12.5 Simultaneous and 3 1 4.5 × 10⁸ 0.7 — 20 (*2 Example 3 continuous irradiation Comparative 43.7 6.3 Simultaneous and 7 1 4.5 × 10⁵ 0.7 — 20 (*2 Example 4 continuous irradiation (*1: Measurement at day 8 of culturing (*2: Measurment at day 6 of culturing

TABLE 2-1 Main culture conditions Culture Temperature Period Green algae Medium device (° C.) pH (day) Light source Example 1 Haematococcus lacustris Modified medium 100 ml 20 7 8 3 in LED NIES-144 strain of BG-11 medium conical flask illumination Example 2 Haematococcus lacustris Modified medium 100 ml 20 7 8 3 in LED NIES-144 strain of BG-11 medium conical flask illumination Example 3 Haematococcus lacustris Modified medium 100 ml 20 7 6 3 in LED NIES-144 strain of BG-11 medium conical flask illumination Example 4 Haematococcus lacustris Modified medium 100 ml 20 7 6 3 in LED NIES-144 strain of BG-11 medium conical flask illumination Comparative Haematococcus lacustris Modified medium 100 ml 20 7 8 3 in LED Example 1 NIES-144 strain of BG-11 medium conical flask illumination Comparative Haematococcus lacustris Modified medium 100 ml 20 7 8 3 in LED Example 2 NIES-144 strain of BG-11 medium conical flask illumination Comparative Haematococcus lacustris Modified medium 100 ml 20 7 6 3 in LED Example 3 NIES-144 strain of BG-11 medium conical flask illumination Comparative Haematococcus lacustris Modified medium 100 ml 20 7 6 3 in LED Example 4 NIES-144 strain of BG-11 medium conical flask illumination 

The invention claimed is:
 1. A method of promoting growth of green algae which promotes the growth of the green algae which is in a state of being a green swarm cell by irradiating the green algae that accumulate astaxanthin with an artificial light, wherein the green algae are grown in a liquid medium while maintaining a state in which the color of a culture solution of the green algae is green or brown by alternately and continuously radiating a red illumination light and a blue illumination light, and a light amount ratio of the red illumination light and the blue illumination light is within a range of 1:1 to 10:1.
 2. The method of promoting growth of green algae according to claim 1, wherein the green algae accumulating astaxanthin are unicellular green algae belonging to the genus Haematococcus.
 3. The method of promoting growth of green algae according to claim 2, wherein the unicellular green algae belonging to the genus Haematococcus are Haematococcus pluvialis (H. pluvialis) or Haematococcus lacustris (H. lacustris).
 4. The method of promoting growth of green algae according to claim 1, wherein a wavelength of the red illumination light is 570 nm to 730 nm and a central wavelength is 645 nm to 680 nm.
 5. The method of promoting growth of green algae according to claim 1, wherein a wavelength of the blue illumination light is 400 nm to 515 nm and a central wavelength is 440 nm to 460 nm.
 6. The method of promoting growth of green algae according to claim 1, wherein a light source of the red illumination light is an LED.
 7. The method of promoting growth of green algae according to claim 1, wherein a light source of the blue illumination light is an LED.
 8. The method of promoting growth of green algae according to claim 1, wherein the light amount of the red illumination light and the blue illumination light is 10 μmol/m²/s to 100 μmol/m²/s at photosynthetic photon flux density in a light irradiation surface of the culture solution. 